TWIP steels may have a stable austenite single phase at room temperature by containing a large amount of manganese and may prevent the movement of dislocations by generating mechanical twins in austenite grains during plastic deformation to be further work hardened. Thus, excellent elongation may be obtained. Since TWIP steels may obtain high tensile strength as well as high elongation, the TWIP steels are materials that may be used as various structural materials.
In particular, ductility of typical ferritic steels may be significantly reduced at a low temperature range, and the reason for this is that when the temperature is decreased to a low temperature range, yield strength rapidly increases to cause brittle fracture.
In contrast, with respect to austenite steels including TWIP steels, since the strength at a low temperature does not rapidly increase as much as that of the ferritic steel, a ductile-brittle transition temperature is generally lower. Thus, the austenite steels including TWIP steels may have potential to be used as a low temperature or ultra-low temperature material.
As a prior art document, Korean Patent No. 1127632 discloses a method of manufacturing a steel strip or steel sheet, as a TWIP steel having excellent ductility at a low temperature, which contains 1.00 wt % or less of carbon (C), 7.00 wt % to 30.00 wt % of manganese (Mn), 1.00 wt % to 10.00 wt % of aluminum (Al), greater than 2.50 wt % and equal to or less than 8.00 wt % of silicon (Si), greater than 3.50 wt % and equal to or less than 12.00 wt % of Al+Si, less than 0.01 wt % of boron (B), less than 8.00 wt % of nickel (Ni), less than 3.00 wt % of copper (Cu), less than 0.60 wt % of nitrogen (N), less than 0.30 wt % of niobium (Nb), less than 0.30 wt % of titanium (Ti), less than 0.30 wt % of vanadium (V), less than 0.01 wt % of phosphorus (P), and iron (Fe) as well as other unavoidable impurities as a remainder. However, the TRIP steel manufactured by this method are only manufactured in the shape of a strip, and also, excellent ductility at an ultra-low temperature of −100° C. or less may not be realized.
Also, Korean Patent Application Laid-Open Publication No. 2001-107473 discloses a TWIP steel including 0.5 wt % to 1.0 wt % of carbon, 10 wt % to 20 wt % of manganese, 4.0 wt % or less of chromium, 0.02 wt % to 0.3 wt % of nitrogen, and iron as well as other unavoidable impurities as a remainder. The above publication is related to the TWIP steel in the shape of a plate and a method of manufacturing the same, and excellent mechanical properties at an ultra-low temperature may also be difficult to be realized in the above alloy.